In U.S. Pat. No. 4,710,570 there are disclosed 3-(phenylamino)-3 H-phenoxizines or phenothiazines of the type whose preparation is the subject matter of the present invention. This patent discloses that these compounds have utility in, photothermographic imaging and more particularly that the "leuko" or reduced form of these dyes are suitable as dye-forming agents in pressure sensitive, thermographic, photothermographic and photographic imaging systems. The '570 patent discloses the preparation of these compounds by the oxidation of phenothiazine with iodine to form a phenothiazin-5-ium perhalide and the subsequent treatment with an amine to yield a 3-(substituted amino)-phenothiazin-5-ium salt. This method suffers from the disadvantage that it provides low yields and mixed products which require extensive purification.
In co-pending application, Ser. No. 471,745 there is disclosed the use of these compounds as mediators for the electrochemical regeneration of the coenzymes dihydronicotinamide adenine dinucleotide (NADH) and dihydronicotinamide adenine dinucleotide phosphate (NADPH). In this patent application, there is described the preparation of the 3-phenylimino phenoxizines and phenothiazines by the reaction of roughly equi-molar concentrations of the appropriate phenoxizine or phenothiazine with the appropriate aniline in the presence of 1 molar silver nitrate in methanol. This method of preparation is not fully satisfactory because it does not work well with all substituted aromatic amines and a silver precipitate must be removed from the product.
Baranov et al report the oxidative condensation of phenothiazine with aromatic amines using ferric chloride as the oxidative coupling agent in Zhurnal Organicheskoi Khimti, Vol. 27, No. 9, 2008 (1989).
In general, there are only a few literature methods for the preparation of substituted 3-phenylamino compounds and they suffer from low yields, the need for extensive purification and poor characterization of the resulting products. Since these compounds are only slightly soluble in organic solvents, large amounts of chloroform (a carcinogen) and costly silica gel are used for purification.
It is an object of the present invention to provide a novel process for the preparation of 3-(phenylamino)-3 H-phenoxazines or phenothiazines which provides high yields while using inexpensive and environmentally safe reagents without extensive purification.